1. Field of the Invention
The present invention relates generally to proximity sensor and, more particularly, to a proximity sensor that directly measures the AC and DC resistances of the sensor coil and, from these measured parameters, derives a discriminator value which exhibits a significantly reduced temperature sensitivity, is a generally linear function of temperature and indicative of the distance between the proximity sensor and a metallic object.
2. Description of the Prior Art
Proximity sensors have been well known to those skilled in the art for many years. One particularly well known type of proximity sensor is the eddy current killed oscillator, or ECKO, which utilizes an oscillator that generates a radio frequency, or RF field, at the face of the sensor. Together, the oscillator and the sensor form a tuned circuit which oscillates at a predetermined frequency. When a metallic object, or target, is moved toward the face of the sensor, eddy currents are established in the target as a result of the oscillating magnetic field. The development of these eddy currents cause the oscillations to diminish in amplitude, or be killed. Typically, an integrator converts the sine wave signal, which is generated by the oscillator, into a DC signal. The DC signal, which varies in amplitude with the amplitude of the oscillator, is sensed by a device, such as a Schmitt trigger, and converted into a digital signal. The digital signal represents the presence or absence of a metallic target in the region of the sensor face.
Sensors of the type described above utilize a threshold detector which can be used to convert the analog DC signal into a digital output which indicates the presence of a metal target. The magnitude of the DC signal varies as a representative function of the distance between the face of the sensor and the target. However, sensors of this type are not commonly utilized to determine the actual distance between the target and the sensor face over a wide range of target distances because of the severe effect that temperature changes have on the accuracy of the sensor output. Because of this deleterious affect of temperature on the DC signal of a proximity sensor, the inability of a proximity sensor to provide a signal that is reliable and repeatable for a wide range of target distances at all temperatures inhibits the use of this type of sensor in a distance measuring application. The severe affect of temperature change of the magnitude of the DC signal therefore prevents proximity sensors of this type from being reliably used to measure the distance between the target and the face of the sensor.
U.S. Pat. No. 4,074,185, which issued to Dardenne on Feb. 14, 1978 describes a method and apparatus for measuring the position of a magnetic rod. An induction coil is fed by means of constant AC current supply and the resulting voltage which appears across the terminals of the coil is measured. The reactive component value of the voltage is determined and used as a representative signal of the position occupied by the magnetic rod in the coil. This apparatus measures the reactive component of the voltage and not the AC or DC resistances.
U.S. Pat. No. 4,509,023, which issued to Heimlicher on Apr. 2, 1985, describes an oscillator with a temperature compensated oscillating coil. It describes a method by which the temperature coefficient of the copper resistance in an oscillating coil is used to compensate for the temperature coefficient of the quality factor of the same coil. An AC voltage that is proportional to the copper resistance of the oscillating coil is generated in the oscillating circuit. The oscillator comprises an oscillating coil which, in turn, comprises a high frequency litz wire. To access the copper resistance of the coil, one of the litz wires is separated from the other wire at a first terminal of the coil and separately connected to a second terminal of the coil. From the second terminal, the coil is seen as a bifilar coil. A constant alternating current source is connected to the second terminal in order to generate an AC voltage across the oscillating circuit that is proportional to the copper resistance of the coil. The oscillator is intended for use in proximity switches which have relatively large switching distances. This device is primarily intended to compensate for the quality factor of an oscillating coil. In addition, the device described in this patent injects a voltage into the separated litz wire which is opposite to that of the oscillator voltage. Furthermore, this patent does not describe a method which measures impedance values.
U.S. Pat. No. 4,591,795, which issued to McCorkle on May 27, 1986, describes a signal conditioning circuit for a linear/rotary variable differential transformer (L/R VDT). The circuit described in this patent can be utilized for ratiometric comparison of L/R VDT primary coil peak excitation and L/R VDT secondary coil peak response. Unique peak detectors are used for detecting both primary and secondary peak values. The circuit may include an oscillator having resistive and capacitive elements in a tank circuit which also includes the L/R VDT primary coil. Offset errors may be determined externally by providing for a momentary shutdown of the oscillator to permit the measurement of DC components to the signal conditioner and excitation DC output. The device described in the McCorkle patent requires that the oscillator be shut down for the period of time required to measure the DC characteristics of the coil. By the nature of this device, it appears to require at least two or three coils and, in the preferred embodiment, utilizes peak detectors.
U.S. Pat. No. 4,893,079, which issued to Kustra et al on Jan. 9, 1990, describes a method and apparatus for correcting an eddy current signal voltage for temperature affects. This device measures physical characteristics of an electrically conductive material by the use of eddy current techniques and compensates measurement errors which are caused by changes in temperature. It includes a switching arrangement connected between primary and reference coils of an eddy current probe which allows the probe to be selectively connected between an eddy current output oscilloscope and a digital ohmmeter for measuring the resistances of the primary and reference coils substantially at the time of eddy current By using this technique, changes in resistance due to temperature affects can be compensated for in determining the true error in the eddy current measurement. The true error can therefore be converted into an equivalent eddy current measurement correction. This device uses a reference and a primary sense coil and the detection method utilizes a balanced bridge scheme.
U.S. Pat. No. 4,942,372, which issued to Heimlicher on Jul. 17, 1990, describes a method and a circuit for reduction of temperature dependance in an oscillator. This circuit, which is generally similar to the other Heimlicher patent described above, reduces the temperature dependance of an oscillator by a circuit that uses two positive feedback paths to provide the positive feedback necessary for maintaining the oscillation of an LC-oscillating circuit. On of the two feedback paths compensates the temperature dependant influences while the other compensates temperature independent influences.
It would be beneficial if a proximity sensor could utilize its single coil in cooperation with a circuit which provides means to directly read the AC and DC resistances of the coil to permit the derivation of a temperature independent discriminator value that can be used to determine the distance between a target and the face of the sensor with sufficient accuracy to confidently and reliably determine the distance between the sensor and the target.